from triple interpolyelectrolyte-metal complexes to polymer-metal nanocomposites

Advances in Colloid and Interface Science 158 (2010) 84–93

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Advances in Colloid and Interface Science

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From triple interpolyelectrolyte-metal complexes to polymer-metal nanocomposites

Alexander B. Zezin a, Valentina B. Rogacheva a, Vladimir I. Feldman a,c, Pavel Afanasiev b, Alexey A. Zezin c,⁎a Department of Chemistry, Moscow State University, Moscow, 119991 Russiab Institut de recherches sur la catalyse et Environnement CNRS, Université de Lyon 1, 2, av. A. Einstein, 69626, Villeurbanne, Cedex, Francec Institute of Synthetic Polymer Materials of Russian Academy of Sciences, Profsoyuznaya ul. 70, Moscow, 117393 Russia

⁎ Corresponding author. Tel.: +7 495 3325836; fax: +E-mail address: [email protected] (A.A. Zezin).

0001-8686/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.cis.2009.09.002

a b s t r a c t
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Available online 30 September 2009

Keywords:Interpolyelectrolyte-metal complexesPolyelectrolytesPolymer-metal nanocompositePreparation of nanoparticlesReduction of metal ions

Nanocomposite polymer materials containing metal or metal oxide particles attract growing interest due totheir specific unique combination of physical and electric behavior. Stoichometric triple interpolyelectrolyte-metal complexes (TIMC) are insoluble in water and in aqueous organic media and may include high contentof metal ions; concentration of ions is easy to vary in such polymeric systems. Reduction of metal ions is acommon method for obtaining nanoparticles. Interpolyelectrolyte complexes reveal high permeability forpolar low-molecular substances and salts. Such swelling behavior is important for the reduction of metal ionsincluded in these solids. The properties of triple interpolyelectrolyte-metal complexes and preparation ofnanocomposites from these materials using various methods of metal ion reduction are discussed in thiswork.

7 495 420 22 29.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842. Triple interpolyelectrolyte-metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

2.1. Interpolyelectrolyte complexes: synthesis and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852.2. Preparation and structure of TIMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3. Synthesis and properties of nanocomposites obtained from TIMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.1. Preparation of nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.2. The structure and properties of IPEC based nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

1. Introduction

Clusters and nanoparticles play a very important role in con-densed-phase physics and chemistry because the typical scale ofvarious physical phenomena determining the properties of materialsis often comparable with the nanoparticle size. Due to the so-calledsize effects, the ultrafine inorganic particles exhibit specific magnetic,optical, conducting and other physical properties. Extremely largespecific surface and uncompensated character of intermolecularinteractions in the interface layer lead to high catalytic activity ofthese species. On the other hand, the huge excess of the surface

energy of ultrafine particles results in their extreme instability inrespect to coagulation and oxidation. For this reason, stabilization ofnanoparticles is one of the major problems. Thus, ideally one needs tocompromise high chemical activity important for many applicationsand long-term stability of nanoparticles. Therefore, the search for themethods of their controlled passivation [1,2] is a challenge from bothfundamental and practical viewpoints.

It is well known [1–4] that macromolecules and polymericmatrices are widely used for the stabilization of nanoparticles. Theprogress in synthesis of macromolecules of various structures allowsone to prepare a wide range of polymer matrices with stablenanoparticles providing a unique opportunity for the design of agreat diversity of functional nanocomposite materials. The nature ofthe functional groups and the structure of polymer chain affectstrongly the stabilization of nanoparticles. Nanoparticles, as a rule, arecharged, and for this reason the macromolecules containing polar

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groups are suitable for their stabilization. In particular, polyelec-trolytes are effective stabilizers of dispersions containing metalparticles [2,4,5]. The most commonly used polyelectrolytes, such aspolyacrylic acid (PAA) and polyethyleneimine (PEI), are often appliedfor the stabilization of nanoparticles in aqueous media [4–6].

Polymer-metal hybrid nanomaterials are potentially useful for awide range of applications, including sensors, catalytic systems,optical materials, elements of microcircuits, etc., so the studies ofstructure and properties of such materials attract a growing interest.

Common approaches to the synthesis and investigation of poly-mer materials with incorporated nanoparticles of metals and/oroxides have been reviewed in a number of monographs [1–3,7]. Cur-rently developed methods for preparation of metal polymer na-nocomposites are usually multi-step techniques, which includesynthesis of metal nanoparticles followed by their embedding intothe polymer matrix.

The controlled reduction of metal ions is one of the widely-usedways to synthesize metal nanoparticles. A routine method for thispurpose is chemical reduction of metal ions. However, due to therelatively low potentials of common reducing chemical agents, thismethod is applicable mainly to nanoparticles of noble metals andsilver or metal oxides [2]. The formation of polydisperse colloids withwide size distribution of particles is a common disadvantage of thisapproach. Furthermore, using inorganic reductants leads to theformation of nanoparticles including impurities, which can be hardlyremoved from the material.

An alternative approach exploits the radiation-induced reductionof metal ions. The application of this approach has demonstrated itsprincipal advantages resulting from the selectivity of radiation–chemical processes and opportunities for their control [8–15]. Use ofthis method makes it possible to synthesize the particles withadjustable sizes and narrow size distribution. Another advantage isthe high reduction potentials of species generated by irradiation,which makes it possible to reduce not only noble metal ions, but alsocopper and nickel particles to metal atoms and, thus, to obtain thecorresponding nanoparticles [8–15].

In addition, in contrast to chemical procedures, the radiation–chemical reduction may yield pure nanoparticles, which is veryimportant for certain applications.

Matrices with the organized polymeric structures are very usefulfor the synthesis of polymer-metal hybrid composites. The polymersystems with ordered supramolecular structure provide new oppor-tunities for the fine control of interface interactions and growth ofnanoparticles [16,17]. 1D, 2D, and 3D metal nanoparticles withvarious degrees of dispersion have been prepared based on polymersystems of various architectures [18–20]. However, implementationof new approaches often leads to the increase in complexity and thecost of nanocomposites, which limit their applications.

In recent years the attention has been focused on working out of asimple single-stage method of synthesis of polymer-metal hybridmaterials directly in the swollen polymer matrices by the reduction ofmetal ions incorporated into hydrogels [21–24]. The systemscomposed of polyacrylic acid and polyethyleneimine form a polymermatrix of interpolyelectrolyte complexes (IPEC) that is suitable for thepreparation of nanoparticles [25,26]. The ions as transitionmetals, canbe introduced into the IPEC to form a new triple interpolyelectrolyte-metal complex (TIMC) with high sorption capacity (up to 20–30 wt.%). It is well known that stoichiometric interpolyelectolyte complexesare swellable in water and exhibit high permeability for small polarorganic substances and salts. The ability of IPEC to swell in water is ofmajor importance for the reduction of metal ions included in thesematrices. As we showed previously, the reduction of metal ions in thematrices of stoichiometric complexes of PAA–PEI can be carried out byboth chemical and radiation-induced reduction. Use of TIMC asmatrices opens new prospects for the synthesis of polymer nano-composites including metal or metal oxide nanoparticles.

The process of preparation of polymer-metal hybrid materials onthe basis of triple interpolyelectrolyte-metal complexes involves thefollowing stages:

– preparation of the IPEC samples;– incorporation of transitions metals ions to form TIMC;– obtaining metal or metal oxide nanoparticles in an IPEC matrix.

In this paper we discuss the methods of preparation of the TIMCsamples, the approaches to synthesis of nanoparticles in IPEC films, aswell as the structure and properties of the resulting polymer-metalhybrid materials.

2. Triple interpolyelectrolyte-metal complexes

2.1. Interpolyelectrolyte complexes: synthesis and properties

IPECs are formed by the addition of reactions between macro-molecules of oppositely charged polyelectrolytes:

The polyelectrolytes contain ionogenic groups that are capable ofelectrolytic dissociation in polar media. They are related to water-soluble polymers.When oppositely charged polyelecrolytes aremixedin aqueous solutions, the new compound (IPEC) is formed immedi-ately [27,28]. Polyanions and polycations in IPEC are bound by the saltbonds between oppositely charged monomer units.

The co-operative character of interpolyelectrolyte interactionmakes IPECs very stable compounds. So they do not dissociate intocomponent polyions in aqueous media. Stoichiometric IPECs incor-porating oppositely charged monomeric units in equal amounts areinsoluble in water due to mutual blocking of ionogenic groups ofoppositely charged polyions from the surrounding water. However, instrongly acidic or strongly alkaline media (as well as in concentratedaqueous salt solutions), the co-operative interaction is broken up. Thedissociation of interpolymer salt bonds results in the dissolution ofIPEC.

In the present paper we have restricted ourselves to the case ofIPEC formed by PAA and PEI. Either of the two polyelectrolytescontains monomer units, which can serve as chelating ligands.Certainly, there is a number of other chelating polyelectrolytes andIPEC capable of forming stable polymer-metal complexes, such as IPECgenerated by PAA and poly(-4-vinylpiridine) and many others.However, PAA–PEI IPEC was studied as chelating polymer compoundin most detail [25,27–31]. Furthermore, it is possible to prepare theIPEC samples in different forms (films, fibers, etc.).

In order to prepare a uniform quality IPEC films it is necessary toprevent the uncontrollable formation of large complex agglomerates,which occurs immediately as a result of the mixing of aqueoussolutions of oppositely charged polyelectrolytes. For this purpose weused [25] the so-called shielding solvents, which suppress thedissociation of weak polyacids and polybases such as aqueous formicacid or aqueous ammonia. 50 wt.% of aqueous formic acid was foundto be optimal as a solvent for PAA–PEI IPEC preparation. 0.5 M solutionof PAA and PEI in this solvent was mixed in equimolar ratio. Thesolutions preparedwere poured out onto a polyethylene film and thendried at the air of ambient temperature. The dried film contained the

86 A.B. Zezin et al. / Advances in Colloid and Interface Science 158 (2010) 84–93

mixture of practically non-interacting free PAA and PEI, and alsoresidual formic acid, which prevents the formation of IPEC. Dry filmswere then washed out in the distilled water up to the neutral рН,which results in the formation of stoichiometric IPEC:

The films formed are uniform, transparent and fragile in the drystate. IPEC films swell in water and the equilibrium swelling index isabout 150wt.% (Table 1). Water is a very efficient plasticizer of IPECsamples, so the stress–stretch curves of swollen IPEC films are similarto those for not vulcanized rubbers (Fig. 1). IPEC films arecharacterized by low values of the initial modulus (~10−2MPa) andhigh relative elongation at break (~500%), and also by the creep. Thesamples of IPEC show high permeability to water and the high dialysispermeability to polar low-molecular organic substances and salts[25,28].

An IPEC formed by weak polyelectrolytes dissociates intocomponents, PAA and PEI macromolecules, both in acidic (рНb3),and in alkaline (рНN11.5) media, as well as in solutions at high saltconcentration. Under these conditions, the decomposition of IPECfilms occurs and finally they are dissolved. The stability of the IPECsamples in such media can be increased by their vulcanization[25,28,30,31]. This can be done by heating the IPEC samples at atemperature above 150 °С resulting in the conversion of interpolymersalt bonds into covalent amide crosslinks:

This reaction proceeds in the solid phase of the glassy IPEC and it ischaractarised by a polychronic kinetics [25,30,31]. The phenomenonof “kinetic stop” is typical for such reactions, so the degree ofconversion can be easily controlled by a proper choice of the processtemperature [25,31]. Table 2 demonstrates the effect of temperatureon the degree of covalent amide crosslinks according to the IRspectroscopic data [30,31], which show that the crosslink concentra-tion can be controlled over a wide range. It is noteworthy that heatingof the glassy IPEC over 200 °C leads to the transformation ofoverwhelming majority of salt bonds (~80 mol.%) into amide cross-links due to ordering in the IPEC precursor [31]. The heat treatment at150–160 °C during 10–15 min produces the films of PAA–PEI IPECscontaining ~5% of interpolymer amide bonds. These films keep theirintegrity in a 0.1 M NaOH as well as in a 0.1 M HCl, in which the initialuntreated IPEC is completely soluble. The stability of IPEC in alkalinemedia is particularly important, because chemical reduction of metalions proceeds in alkalinemedia (for example, in the case of commonlyused borohydrides of alkaline metals). In addition to the stabilizationeffect, the vulcanization of IPEC films leads to their considerablemechanical strengthening [25].

2.2. Preparation and structure of TIMC

Coupling of PAA and PEI macromolecules enhances their ability toserve as ligands for many metal ions, so the triple polyaion–

polycation–metal complexes reveal extremely high stability. Thesorption ofmetal ions from aqueous solutions of salts by the IPEC filmswas used to prepare TIMC samples. Interaction of IPECwithmetal ionsimplies embedding of these ions between PAA and PEI macromole-cules and formation of the triple complexes:

To prepare the TIMC samples, IPEC films were immersed intoaqueous solutions of transition metal salts. It was found that the TIMCbound the ions of the transition metals extremely strongly [25,28,29].For example, the PAA–PEI IPEC films were able to adsorb the Cu2+

ions from Сu (NO3)2 aqueous solutions at salt concentration as lowas 10−5M. It is also the case for othermetals, for example, Co(II), Ni(II),Fe(III), whichmakes it possible to use IPEC for extraction of metals fromthe dilute aqueous solutions, and for ion-exchange concentration ofmetals for analytical purposes. Reaction (3) demonstrates that thesorption of metal ions by complexes results in a decrease of pH of thefilm environment. In order to compensate this effect, it is necessary toadd alkali for shifting the equilibrium, which allows one to prepare theTIMC films with relatively high content of metal ions.

The ligand environment ofmetal ions includes functional groups ofboth macromolecules of PAA and PEI. Reaction of complex formationleads to coloration of initially colorless IPEC films, as a result ofincorporation of metal ions. For example, in the case of Cu(II), thefilms become deep dark blue, whereas incorporation of Ni(II) leads togreen coloration. Thus, coloration of IPEC is a qualitative indicator oftransformation of IPEC to TIMC.

The application of EPR spectroscopymakes it possible [29] to studythe structure of complexes of paramagnetic Cu2+ ions with PAC andPEI. It has been shown, that copper ions in IPEC matrices may occur intwo kinds of ligand environment, namely four NH group (structure A,reaction 4) or two NH and two carboxylate group (structure B) . Theratio of A and B structures for these complexes in IPEC matricesdepends on the proportions of reagents and solution pH value [29]. Itis worth mentioning that the films with partially vacant ligand sitesmay include Na+ ions resulting from alkali titration.

Table 3 demonstrates high sorption capacity of IPEC for metal ions.The largest capacity was found for Cu2+ions, which is understandable,because the geometry of IPEC fits well with that of the initial complex.The ionic-exchange capacity in this case is 8.6 mg-ekv Cu2+ on 1 g ofdry IPEC, i.e., actually very close to that expected for structure B(reaction 4 ) of complex PAA–PEI–Cu2+ with fully occupied ligand

Table 2Effect of the temperature on the content of covalent amide crosslinks.

T °C Crosslinks amount, %

160 5180 15200 30

Table 1Swelling index of PAA–PEI interpolyelectrolyte complexes (IPEC) and triple inter-polyelectrolyte-metal complexes (TIMC).

Samples IPEC non-crosslinked

IPECcrosslinked

TIMC, 10 wt.%Ni2+, non-crosslinked

TIMC, 10wt.% Ni2+,crosslinked

TIMC, 20wt.% Ni2+,crosslinked

160 44 42 45 36


sites. Similar distribution is characteristic for other metals under theconditions of saturation. The analysis of the EPR spectra revealsgradual increase in linewidth with increasing concentration of copperions in the IPEC films. Thus, the character of the obtained spectra doesnot bear any evidence for high local concentration of copper ions,which implies an uniform population of vacant lattice sites withmetalions. Actually, TIMC can be considered as the precursors for metalpolymer hybrid materials, so characteristics of the resulting nanoma-terials are essentially determined by their structure and properties.The homogeneous structure of TIMC with variable concentration ofmetal ions provides an opportunity for obtaining nanocompositeswith controlled content of metal nanoparticles. Transformation ofIPEC to TIMC leads to appreciable decrease of swelling ability samplesin water (Table 1) and, accordingly, substantial increase of durabilityof films. It considerably simplifies the procedures of reduction ofmetal ions in TIMC and preparation of nanocomposites.

3. Synthesis and properties of nanocomposites obtainedfrom TIMC

3.1. Preparation of nanocomposites

The films of triple polymer-metal of complexes are swellable inwater and aqueous organic media and they possess high permeabilityfor polar low-molecular substances and ionic salts (as well as IPEC).These properties make it possible to reduce the metal ions directly inpolymeric matrix by using chemical and radiation approaches. It hasbeen shown that the reduction of silver, palladium, copper, nickel andiron ions may occur in the samples of complexes. In the case ofcomplexes with Fe ions, the IPEC nanocomposites with iron oxidenanoparticles were obtained by exchange reactions [32]. KeepingTIMC films in NaOH solution leads to the appearance of nanoclustersof iron hydroxide and subsequent drying of films at 60–70 °C resultsin the formation of ion oxide nanoparticles.

PAA–PEI–Fe3þþ OH

−→FeðOHÞ3 in PAA–PEI matrix→Fe2O3 in PAA–PEI matrix

ð5Þ

Fig. 1. Dynamic mechanical characteristics of (1) crosslinked and (2) non-crosslinkedIPEC films swollen in aqueous media.

Formation of metal nanoparticles in the process of metal ionreduction occurs, as a rule, through the formation of short-livedtransient clusters. Synthesis of nanoparticles in aqueous solutions inthe presence of macromolecules containing polar and ionogenicgroups makes it possible to control the nanoparticle size and providestheir stabilization. The macromolecules PAA or PEI with links ofchelation for metal ions are often used as additives of such kind. Thegeneral scheme of reduction may be presented for complexes ofbivalent metal ions as follows:

½LigandsMe2þ�→½LigandsMe

þ�→Me0 ð6Þ

Me0 þ ½LigandsMe

2þ�→½LigandsMemþn � ð7Þ

The studies using time-resolved methods (in particular, pulseradiolysis) have demonstrated the formation of metastable clusterswith a particular structure (“magic clusters”). For example, formationof silver nanoparticles occurs through the intermediate formation ofAg42+ and Ag82+, and, in the case of copper, the clusters Cu22+ and Cu4

2+

were found [9,10]. The present scheme demonstrates that reductionof ions to metal atoms (reaction 6) is a necessary step for the forma-tion of the metal nanoparticles. Such processes of formation of iso-lated atoms are often characterized by high values of redox potentials(e. g., –1.8 V for Ag+/Ag0 and –2.7 V for Cu+/Cu0 [9,33]).

The chemical reduction of metal ions in a liquid phase is the mostcommonly used approach for the preparation of nanoparticles inaqueous and non-aqueous environments due to simplicity of theprocedure. A number of chemical substances (aluminum hydrides,borohydride, hypophosphites, formaldehyde, salts oxalic and tartaricacids, hydroquinone, hydrogen, hydrazine, etc [2,3]) can be used asreducing agents. Complete reduction usually requires multiple excessof a reducing agent. Generally, this method is applicable for thesynthesis of nanoparticles of silver and noble metals. The nanopar-ticles obtained by chemical reduction often contain impurities [2,3].Formation of polydisperse colloid with wide size distribution ofparticles is a typical drawback of the chemical approach. However,chemical reduction in the presence of polymer additivesmay yield thenanoparticles of adjusted sizes. The presence of nanostructures inpolymer systems gives fine instrument for control of growth ofnanoparticles by varying the interface interactions [16]. The nano-particles of noble metals with sizes in the range of 2–6 nm weresynthesized by chemical reduction in themicelles of block copolymers[16,17,34,35].

Spherical polymer brush particles were used as templates forchemical reduction of metal ions and immobilization of gold,platinum, palladium or silver nanoparticles in aqueous solutions[36–39]. Stable nanocomposites based onmicrogels with hydrophobic

Table 3Sorption characteristics of IPEC.

Metal Ion-exchange capacity, wt.% Ion-exchange capacity, mg-eqv/g

Cu(II) 27 8.6Co(II) 20 6.8Ni(II) 20 6.8Fe(II) 6 3.0Ag(I) 22 2.1

Fig. 2. UV–vis-spectra of the PAA–PEI–Cu2+films: (1) before and (2) after reduction.

Fig. 3. XPS data on the PAA–PEI–Cu2+films after chemical reduction.


core and hydrophilic shell with sizes of nanoparticles from 2 to 9 nm[40] were obtained for different metals. The metal nanoparticles werealso sythesized in “nanoreactors” of dendrimer molecules [41,42].Noble metal nanoclusters have been prepared in microgels of N,N-dimethylacrylamide-based crosslinked polymers [43].

Original synthesis of silver and palladium nanoparticles inmultilayer polyelectolyte films for the preparation of nanocompositematerials was suggested by Bruennig et al. [44]. The size-controlledsynthesis of nanoparticles of noble metals by chemical reduction wascarried out in gels of crosslinked polymers with nanopores of variablesize [24]. The in situ method of generation of metal nanoparticles infilms of polymer hydrogels was developed by Radhakrishnan et al.using poly(vinyl alcohol) acts as a reducing agent, stabilizer andimmobilizer [22,23]. It is noteworthy that this technique is biologi-cally and environmentally friendly. The process allows efficientcontrol for particle size and size distribution, however, it is limitedto the preparation of nanoparticles of noble metals and silver.Nanocomposite material with metallic nanoislands embedded in asemiconducting matrix was obtained by the synthesis of Ag, Au, andPt nanoparticles using a water-dispersible conducting polymercolloids composed of polyaniline [45].

The radiation–chemical reduction of ionic species is a promisingtechnique for the preparation of nanoparticles in multicomponentsystems due to high efficiency and specific selectivity of the radiation-induced chemical processes. This method does not require any specificchemicals (reducing agents), allowing to get high-purity nanoparticles.In this case, the ion reduction is due to the formation of reducing agentsin chemical systems under irradiation. The absorbed dose of radiation(D) and the dose rate (P) are the most important quantitativeparameters, which characterize the radiation effects of in materials.The dose is defined as the energy of an ionizing radiation absorbed in amass unit of substance, and the dose rate is the dose absorbed per timeunit. Generally speaking, the total number of active species formedunder irradiation is proportional to the absorbed dose, and the rate offormation of active species depends on the dose rate. Using ionizingirradiation allows one to obtain the particles with adjusted size andnarrow size distribution. The variation of the dose rate [8] made itpossible to get metal clusters and nanoparticles with sizes of 1–50 nm.Also to be mentioned, in general, this method of reduction is a way toproduce nanoparticles in solids and at low temperatures. However, forpractical purposes, the radiation-induced synthesis of metal nanopar-ticles is mainly carried out in aqueous solutions.

In this case, the reactive species are formed by water radyolisys:

H2O→e−aq;OH

:;H;H2;H2O2 ð8Þ

One of the basic products of water radyolysis is hydrated electron,which may act as the most powerful reducing agent for metal ions(redox potential of 2.8 V [46]). Therefore, the radiation–chemicalreduction makes it possible to prepare nanoparticles from many metalions (not only of noblemetals). The synthesis of copper nanoparticles isan illustrative example of the advantages of the radiation–chemicalmethod, because irradiation of Cu2+ solutions leads to formation ofmetal nanoparticles in the pH range of 5–9 [9], while use of chemicalreduction, as a rule, leads to formation of copper protoxide. The γ-irradiation of aqueous solutions of polyethyleneimine–Cu2+complexeswas used for preparation of metal nanoparticles with an average size of4 nm [9].

Recently, significant attention was paid to the possibility of usingnatural polymers for the synthesis of nanocomposites. In particularnumerous efforts were made to synthesize metal nanostructures insolution or suspensions of chitosan using chemical [47–50] electro-chemical [51] or radiation [52,53] approaches since this naturalpolysaccharide may act as a “green” stabilizing agent for ultrafineparticles. These studies were mainly simulated by antibacterialproperties of chitosan and obtained composites.

Both chemical and radiation-induced reduction of metal ions can beapplied for preparation of nanoparticles in the films of PAA–PEIcomplexes. Various approaches for reduction of metal ions in TIMCand preparation of nanocomposites were investigated inmost detail forcomplexes of PAA–PEI–Cu2+ and PAA–PEI–Ni2+ [25,26,54]. Sodiumborohydride was used for chemical reduction of metal ions in the TIMCsamples. In this case, swollen TIMC films of 100÷500 mcm thicknesswere immersed into an aqueous solution of sodium borohydride. Themolar ratio of a reducing agent to metal ions [NaBH4]/[Ме2+] wastypically ca. 2. The evolution of gaseous hydrogen and change of filmcolor in the process of reduction of metal ions was observed. Thereaction occurs in the alkaline environment owing to partial hydrolysisof sodium borohydride (рН=11). The reduction was carried out in themixture of water with isopropanol at room temperature.

After a chemical reduction, initially dark blue films of complexesTIMC·Cu2+ get brown coloring, characteristic for metal oxidenanoparticles. The optical spectra of PAA–PEI–Cu2+

film (1) and thesame film after chemical reduction of metal ions (2) are shown inFig. 2. An absorption maximum at ca. 230 nm characteristic for Cu(I)state is clearly observed in the spectrum of the reduced film. An XPSstudy of the sample (Fig. 3) of chemically reduced PAA–PEI–Cu2+

demonstrates the formation of Cu2O. The X-ray scattering study alsoreveals the presence of protoxide particles, which indicates formationof IPEC, filled with Cu2О. The process of reduction of Cu2+ incomplexes may be schematically described by the reaction:

NaBH4 þ PAA–PEI–Cu2þ þ H2O→Na3BO3 þ PAA–PEI–Cu

þ þ H2↑ ð9Þ

It is worth noting that the redox potential of NaBH4 is −1.24 V[22]. Redox potential for Cu2+/Cu+ is −0.15 V, and that of reductionof Cu+ to metal atoms is −2.7 V [9]. For this reason, a chemicalreduction of Cu2+ leads to the formation of Cu+ and the formation ofcopper protoxide occurs in an alkaline environment.


The radiation–chemical approaches also have been applied forreduction of copper ions in the PAA–PEI–Cu2+ complexes using X-rayand γ-radiation [25,26,54]. Swollen films were irradiated at roomtemperature in the absence of oxygen air in a water–ethanol mixturecontaining 10 wt.% of alcohol. As mentioned above, in this case, thereducing agents result mainly from radiolysis of water–alcoholenvironment (reaction 8). The irradiation of aqueous solutions resultsin formation of both reducing species (hydrated electrons, H-atoms)and OH radicals, which are strong oxidizers. OH radicals may reactwith macromolecules yielding crosslinking or chain scission. In orderto prevent the metal atoms from oxidation, it is practicable to usealiphatic alcohols as the scavengers of OH radicals (e.g.). The ethanolreacts with OH radicals by forming radicals:

UOHþRCH2OH→R·CHOHþH2O ð10Þ

The radiation-induced reduction of copper ions in the binarycomplexes PEI–Cu2+ in homogeneous dilute water–alcohol systemwas investigated in detail by Ershov [9]. The process of ion reductionin solutions of complexes occurs through intermediate formation ofcomplex PEI–Cu1+:

PEI–Cu2þ→PEI–Cu

1þ→Cu0 ð11Þ

It is logical to assume a similar two-stage mechanism of reductionof copper ions in the case of PAA–PEI complex. Redox potential of theCH3

·CHOH radical is ~1.5 V [33], so these species may reduce Cu2+ toCu+. Reactive species formed under irradiation of a polymeric matrixalso can take part in the redox processes.

Analysis of difratograms [54] of irradiated samples shows thepresence of reflexes indicating the inter-plane distances of 2.09±0.01and 1.81±0.01 Å (the corresponding parameters for a copper crystallattice are 2.08 and 1.81 Å [55]). This result proves that the radiation–chemical reduction of Сu2+ ions in TIMC leads to the formation ofmetal particles of copper. The spectrum of electron energy loss (Fig. 4)obtained by electron microscopy also demonstrates complete trans-formation of Сu2+ to metal copper.

The EPR study of the radiation–chemical reduction allows one toinvestigate the kinetic features of this process and the effect ofcomplex structure on transformation of copper ions. The Cu2+ ion isparamagnetic, so EPR spectroscopy can be used both for monitoring ofconsumption of parent paramagnetic cations and for getting directinformation about the ligand environment in complex systems[26,54].

Fig. 4. Spectrum of electron energy losses of the PAA–PEI–Cu2+ film after irradiation.

The dose dependence of decay of the EPR signal from Cu2+ ionsupon γ- [26] and X-ray [54] irradiation of the PAA–PEI–Cu2+

complexes made it possible to calculate the radiation–chemical yieldsof reduction of copper ions. The corresponding values were found tobe above 30 ions per 100 eV of the energy absorbed in the swollen filmin the case of γ-irradiation and N100 ions/100 eV in the case of X-rays.These values are much higher than those obtained in dilute aqueoussolutions of the PEI–Сu2+ complexes, where the yield of reduction ofcopper ionswas found to be only 6.2 species per 100 eV [9]. Actually, thetotal radiation–chemical yield of the active species capable to reduce ofCu2+ to Cu+ (eaq – H-atoms, alcohol radicals and radicals from apolymeric matrix) is several times smaller than the yield of Cu2+

consumption observed in irradiated swollen films. The most probableexplanation of extremely high yields of reduction of copper ions isconcerned with involvement of the reducing species generated in theouter environment. In particular, acetaldehyde, one of the principalstable products of the radiolysis of aqueous ethanol solutions [56], mayact as a reducing agent for the Cu2+ ions. This explanation implies thattheprocesses of an interphase exchange of reducing agents between theswollen film and the outer aqueous-alcoholic environment result intransformation of metal ions incorporated in the films. We may notethat thehigh radiation–chemical yieldsof reductionof ionsdemonstrategood prospects for use of the IPEC matrices for the radiation–chemicalsynthesis of nanoparticles by this technique.

The quantitative analysis of EPR spectra shows that the reductionrate is independent of the initial content of the copper ions in thesamples irradiated with γ-rays [26]. Therefore, the time of half-decayof Сu2+ ions is proportional to initial content of metal ions in the TIMCfilms. In contrast with this observation, in the case of X-rays, thereduction rate increases with increase of the initial concentration ofСu2+ ions in the samples [54]. This result can be explained bydifference in basic mechanisms of radiation interaction with matterfor γ-rays and X-rays. In the former case (γ-rays with the energy of ca.1 MeV), the energy absorption is mainly due to the Compton effect,and the absorption coefficient is virtually independent on atomicnumber of the absorber material (Z). However, in the case of X-rays(E=20–50 keV), the dominating mechanism is photo effect, whichreveals a very strong Z dependence on the absorption coefficient (forexample, the mass absorption coefficients for the photons withE=30 keV are 0.145 and 9.42 g/cm2 for water and copper, respec-tively [57]). This leads to effect of concentration of metal ions in IPECon the reduction rate, because the metal ions have much larger Zvalues than the atoms of organic film and aqueous environment. Forthis reason, the efficiency of reduction increases for the films withhigher metal ion content. From a practical point of view, this resultsuggests good prospects for using X-ray irradiation for obtainingmetal polymer hybrid materials from the systems with rather high(10–30wt.%) content of metal ions.

The magnetic resonance parameters of complexes of Cu2+ ionswith PAA and PEI were analyzed in detail previously [29]. The IPECmay include Cu2+ ions in different ligand environment: (with four NHgroups (I)), with four NH groups and salt bonds (no coordination) (II),with two carboxylate groups (III), and with two NH groups and twocarboxylate groups (IV). Analysis of EPR spectra [26,54] hasdemonstrated that irradiation results not only in a decrease of signalintensity, but also in the change of proportion of copper ions trappedin various ligand environments. Possible explanation of this effectmay be concerned with different availability of Cu2+ ions in variousligand environments for the attack of reducing agents.

The Ni(II) ions in the TIMC samples also were reduced by chemicaland radiation–chemical methods. In this case, the limiting stage isreduction of Ni(II) to Ni(I), because it is characterized by a higherredox potential [10]. An opportunity for obtainingmetal nanoparticlesof nickel by chemical reductionwas reported previously [58]. Thefilmsof complexes PAA–PEI–Ni2+ got dark brown coloration after reductionby sodium borohydride [25]. The spectrum of electron energy losses

Fig. 7. Microdifractograms of PAC–PEI–Ni2+ complex after the radiation-inducedreduction of Ni2+ ions (initial content 6 wt.%; irradiation dose 40 kGy). Reflexes 1, 2, 3,and 4 correspond to inter-plane distances of 2.03; 1.76; 1.24, and 1.06 A

´ , respectively.

Fig. 5. Spectrum of electron energy losses of the PAA–PEI–Ni2+ film after irradiation.


corresponds to known spectrum of metal nickel, (Fig. 5) and theanalysis of microdifractogram shows the presence of metal particles[25]. Thus, reaction (12) results in reduction of Ni2+ to metal:

NaBH4 þ PAA–PEI–Ni2þ þ H2O→Na3BO3 þ Ni

0 þ H2↑ ð12Þ

Reduction of Ni2+ in the TIMC complexes in water–ethanolmixtures by radiation–chemical method has also been demonstrated[25,26]. The irradiation leads to a dramatic decrease in the intensity ofthe optical absorption of Ni2+ ions (bands at 670 and 730 nm) (Fig. 6).Microdifractograms of the irradiated samples show formation ofmetal nanoparticles (Fig. 7).

An attempt was also made to characterize the products formed atvarious stages of irradiation by EPR. However, the signals of Ni+ (anintermediate product of a single-electron reduction) were not observedin the samples irradiated up to the doses in the range of 80–320 kGy.These results show that Ni+ is not stabilized in the matrix in noticeableamounts and the formation of Nio and further aggregation to Ninanoparticles may occur rapidly due to disproportionation of the Ni+

ions:

PAA–PEI–Ni2þ þ e

−aq→PAA–PEI–Ni

þ→Ni0 þ PAA–PEI–Ni

2þ ð13Þ

Change of optical properties after irradiation of TIMC hasdemonstrated reduction in swollen films also palladium and silverions [59,60]. In general, one may conclude that both chemical andradiation–chemical approaches provide effective reduction of metalions directly in TIMC matrices, which leads to formation of stabilizednanoparticles. However, chemical method is applicable only for

Fig. 6. Optical spectra of PAA–PEI–Ni2+ complex (1) before irradiation and (2) afterirradiation to the dose of 320 kGy.

obtaining nanoparticles in thermally vulcanized (crosslinked) IPECmatrices. Poor regularity of chemical processes probably causesdestruction of non-crosslinked matrices. In contrast, the use ofirradiation allows one to prepare nanocomposites from both cross-linked and non-crosslinked films of IPEC due to specific control of theradiation–induced processes.

3.2. The structure and properties of IPEC based nanocomposites

Themethod of synthesis naturally affects the structure and sizes ofthe nanoparticles obtained from TIMC. The exchange reactions makeit possible to obtain metal oxide particles (Section 3.1). The reductionof metal ions in PAA–PEI complexes leads to formation of both metaland oxide nanoparticles. The behavior of nanocomposites andprospects of their application depend on the electronic nature ofnanoparticles, size, volume fraction and their spatial distribution.Polyelectrolyte materials including ultrafine particles of silver showantibacterial properties, so they are used in medicine. Anotherpossible application is concerned with the preparation of catalyticmaterials with metal and semiconductor particles for variousapplications, including fuel cells [1–3]. In this case, the accessibilityof particles for reagent transport in catalytic processes is an importantissue. The solutions of micelles of block copolymers with metalnanoparticles demonstrated catalytic activity [16] in reaction ofhydrogenation and oxidation of organic compounds. Nanoparticlesof palladium show catalytic activity in reaction of selective hydroge-nation of unsaturated alcohols [44,61]. The catalytic materials werealso obtained by synthesis of palladium nanoparticles in functionalresins [24]. Polyelectrolyte brushes may act as carriers for metalnanoparticles in heterogeneous hydrogenation reactions [39,40].

Nanostructured materials are important candidates for photonicdevices, band-pass filters, components of nonlinear optical systems,optical limiters etc [1–3]. The optical properties of polymer-metalcomposites obtained by pulsed laser deposition were discussed in[62]. The optical limiting characteristics for silver nanoparticle-embedded polymer films of poly (vinyl alcohol) were discussed[22,23]. The materials with semiconductor nanoparticles have pro-spects for the design of solar battery. The nanowires and elements ofmicrocircuit chips are a topical field of application for metal andsemiconductor nanostructures.

Nanoparticles of magnetic oxides show particularly superpara-magnetic behavior. The polymer inorganic hybrid material includingparticles of iron oxide was synthesized in IPEC matrix for the study ofmagnetic properties [32]. Alternative approaches were used for

Fig. 8. Magnetization of nanocomposite sample 1 under (a) T=81 K and (b) 291 K.

Fig. 9. TEMmicrograph of irradiated film of the PAA–PEI–Cu2+ complex (initial contentof Cu2+ 6 wt.%; irradiation dose 320 kGy).


preparation of nanoclusters. The particles of ion oxide were preparedthrow exchange reactions (sample 1) (Section 3.1.) or by reduction ofiron ions and further oxidation of obtained metal nanoparticles inPAA–PEI films (sample2). Analysis of the low-temperature Mossbauerspectra show the presence of iron oxide particles in a polymer matrixfor both methods of preparation. The magnetization curves at thetemperatures between 81 K and 291 K (Fig. 8) reveal dependencecorresponding to the superparamagnetic state for sample 1. In thecase of sample 2, the magnetic susceptibility shows linear fielddependence in the whole range studied. The difference in magneticbehaviormay be explained by variations in interaction of nanoclusterswith polymer matrix for the samples obtained by alternativeapproaches [32]. Strong interaction of oxide particles of sample 1with polymer matrix after alkaline solution treatment may result indecrease of surface tension while the process reduction of metal ionleads to appearance of unconfined space and decrease the interactionbetween clusters and matrix.

Table 4Electric conductivity of complexes and nanocomposite films.

Film Conductivity, Ω−1cm−1

IPEC 6.10−10

PAA–PEI–Cu2+ 6.10−7

PAA–PEI–Cu2O 3.10−5

PAA–PEI–Ni 3.3

Structure and properties of inorganic polymer hybrid materialsformed in TIMC depend on away of reduction of metal ions [25,26,54].The chemical reduction of copper ions (Section 3.1.) in the TIMCmatrices leads to formation of particles of copper protoxide. Themicrograph of the TIMC film [25] demonstrates that the samplesinclude the nanoparticles with the size of an order of 10 nm. Theelectric conductivity of the materials with the copper protoxidenanoparticles is very low (Table 4) (~10−5Ω−1 cm−1) in comparisonwith the similar materials including particles of metal nickel (whichwill be considered in detail later).

In the presence of air oxygen the brown color of swollen samplesof the nanocomposite, is changed into initial dark blue characteristicfor the Cu2+ complexes. Thus, the particles of Cu2О are rapidlyoxidized to Cu2+ and the structure of the initial TIMC is recovered.Hence, the Cu2О nanoparticles demonstrate high chemical activity,which should be attributed to their small sizes. The formation ofnanocomposites including Cu2О leads to a five-fold lower ionic-exchange capacity of the Cu2+ ions in comparisonwith the initial IPECfilms. The effect of dramatic decrease of ionic-exchange capacityshows, that a considerable fraction of PAA and PEI links is blockedbecause of interaction with nanoparticles Cu2О playing a role of anactive filler. The stress–strain curves obtained by uniaxial extension inaqueous media demonstrate considerable increase of elastic modulusdue to strong interaction of Cu2O nanoparticles with the IPEC matrix[25].

Irradiation of the PAA–PEI–Cu2+ complex leads to formation ofreddish-brown film with metal tint. The nanocomposites including

Fig. 10. Microdifractogram of the PAC–PEI–Cu2+ complex after radiation-inducedreduction of Cu2+ ions (initial content 6 wt.%; irradiation dose 320 kGy). Reflexes 1 and2 correspond to the inter-plane distances of 2.01 and 1.19 A

´ , respectively.

Fig. 11. Microdifractograms of the PAC–PEI–Ni2+ complex after chemical reduction of Ni2+: (a) fragment with nanoparticle size of 2–5 nm, (b) fragment with nanoparticle sizeN10 nm. Initial content of Ni2+ in the film 10 wt.%. Reflexes 1, 2, and 3 correspond to the inter-plane distances of 2.02 A

´ , 1.16 A´ , and 2.83 A

´ , respectively.


the particles of metal copper are formed in this case (Section 3.1.). Thetypes of radiation source and conditions of irradiation effect onstructure of resulting materials. The micrographs (Fig. 9) measured byTEM for γ-irradiated samples [25,26] reveal the presence ofnanoparticles with an average size of 2.5 nm and a rather narrowsize distribution. The electronic microdifractogram (Fig. 10) demon-strates the broad reflexes indicating the small sizes of the obtainedparticles. The reflexes correspond to the inter-plane distances of 1.19and 2.00 Å, which demonstrates the formation of metal particles (tobe compared with known values of 1.09; 1.28; 1.81 and 2.08 Å [55]).Note that wide reflexes corresponding to 1.81 and 2.08 Å are notresolved and look as a single reflex corresponding to the inter-planedistance of ca. 2.00 Å. Similar effect is observed for reflexescorresponding to 1.09 and 1.28 Å (Fig. 10). The analysis of the EPRdata demonstrates oxidation of nanoparticles in the swollen samplesin the presence of oxygen air and regeneration of structure of PAA–PEI–Cu2+ complex for 3–5 days. However, the dry samples ofnanocomposites demonstrate high stability.

The application of X-ray irradiation yields nanoparticles with widesize distribution [54]. The micrographs of films demonstrate thepresence of both small particles (2–5 nm) and relatively largeparticles (N10 nm). The superposition of narrow reflexes of largenanoparticles and diffuse signal of small particles are clearly observedin the difractograms. The narrow reflexes [59,60] correspond to theinter-plane distances of 1.09; 1.28; 1.81 and 2.08 Å in the lattice of

Fig. 12. TEM micrograph of triple metal polymer complexes PAA–PEI–Ni2+ film

metal copper [55]. As mentioned above, in the case of X-ray radiation,the rate of ion reduction increases with increasing of metal ionsconcentration in polymer matrix [54]. In the case of X-rays, theformation of clusters and nanoparticles results in the formation oflocal regions with microheterogeneous distribution of active species,whichmay lead to a non-uniform size distribution of nanoparticles, incontrast to γ-irradiation. For this reason, the initial content of copperions affects the sizes of the resulting nanoparticle and their sizedistribution.

Reduction of nickel ions by different methods leads to thepreparation of metal nanoparticles (Section 3.1.). The magnetisationcurves for dry films of obtained nanocomposites qualititativelyresemble that of metal nickel [25]. This result can be considered as afurther proof for the presence of metal nickel in the films. The analysisof microdifractograms shows the structural heterogeneity of materi-als formed by chemical reduction of nickel ions (Fig. 11). The broadreflexes observed in the microdifractogram of sample areas, whichcontain the particles of ca 2.5 nm in diameter. The characteristicnarrowing of diffuse bands and appearance of isolated diffractionspots on the microdifractograms of selected areas in some samplesconfirms the presence of bulk metal in the chemically treatedsamples. Since the probing electron radiation has a wavelength of0.04 Å, the appearance of diffraction grains demonstrates that the sizeof nickel particles is larger than 10 nm. Analysis of micrographs showsthe presence of the regions of nanocomposite with different sizes of

after the chemical reduction of Ni2+ ions (initial content of Ni2+ 10 wt.%).


nanoparticles (2–5 and ca. 40 nm, respectively) (Fig. 12). Additionalreflexes (the most intense one corresponds to 2.8 Å, Fig. 11b) areprobably due to the presence of some impurities in the chemicallyreduced samples.

Table 4 demonstrates that this metal polymer hybrid materialshows relatively high electric conductivity. The ionic-exchangecapacity of obtained nanocomposite is several times lower incomparison with that of initial IPEC. This large effect demonstratesthat nickel nanoparticles strongly interact with IPEC matrix as well asin the case of IPEC–Cu2О nanocomposite.

In contrast with chemical approach to the nickel ion reduction, γ-irradiation makes it possible the preparation of material with uniformsize distribution [25,26]. The dimensions of nanoparticles vary from1.8 to 4.5 nm. The broad reflexes observed in the microdifractogram[25] confirm the presence of particles with the size smaller than10 nm.

The effect of the sample composition on size and size distributionwas observed for nickel and copper particles prepared by X-rayirradiation. However, in the case of nickel, the formation ofnanoparticles occurs mainly in the bulk of PAA–PEI–Ni2+ complexfilms in aqueous alcohol mixtures. The difference in mechanisms ofion reduction and formation of nanoparticles was found for copperand nickel ions [9,10]. The reduction of nickel ions occurs throughdisproportionation (Section 3.1) and low mobility of generatedclusters may lead to effective formation of particles in the bulk ofthe IPEC films.

The swelling coefficient of composites obtained inmatrices of non-crosslinked complexes of PAA–PEI–Ni (10 wt.% of Ni) is apt 100%. It ismore than for PAA–PEI–Ni complex (Table 1), but is less than one fornon-crosslinked complexes of PAA–PEI. This result indicates interac-tion between nanoparticles and polymer matrix and show thatultrafine particles play the role of active filler. The behavior ofcrosslinked samples is determined by thermally formed amide bonds.It is worth noting that the swelling ability of polymer matrix does notchange appreciably as a result of irradiation (Table 1).

4. Concluding remarks

In summary, we may notice that the PAA–PEI complexes may actas effective systems for sorption of various metal ions from solutions.The ion-exchange yields triple interpolyelectrolyte-metal complexes,which may incorporate very high concentration of metal ions (up to30 wt.%). This procedure makes it possible to get materials withvariable content of metal ions homogeneously distributed in the bulkof the samples. The PAA–PEI complexes can be prepared in the form offibers, films or blocks and they can be applied on porousmaterials, likecoal or silica gel.

The samples of triple polymer-metal complexes possess highpermeability for small polar molecules and ionic salts. Theseproperties allow one to obtain inorganic nanoparticles directly inthe polymeric matrix by means of reduction or other chemical andradiation-induced processes. Thus, the interpolyelectrolyte com-plexes have can be considered as universal matrices for synthesis ofinorganic polymer hybrid nanocomposites. The approaches discussedin this review make it possible to obtain both metal and metal oxidenanoparticles of different structures and provide wide opportunitiesfor synthesis of nanocomposites with controlled content, size andspatial distribution of nanoparticles.

Acknowledgement

This workwas supported by Russian Foundation for Basic Research(grant No.09-03-00877а).

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